Aerial Damper Changeout On Energized 500-kV DC Line

Maintaining continuous operation of any transmission line is important, but the concern is even more critical when discussing an extra-high-voltage (EHV) line. That is why Manitoba Hydro (MH; Manitoba, Canada) moved quickly when it ascertained that spacer-damper units were failing on its Henday Radisson-Dorsey 500-kV dc line. While the line remained energized, MH refurbished these units and installed additional dampers.

MH operates two 558-mile (898-km) ±500-kV dc guyed tower lines (Bipole 1 and 2) between the Radisson Converter Station in northern Manitoba and the Dorsey Converter Station in southern Manitoba. Each line consists of a positive and negative pole strung with twin bundled 1843 kcmil 72/7 ACSR Nelson conductors using spacer-dampers to maintain a separation of 1.5 ft (0.6 m) between subconductors. In the original design, seven spacer-dampers, supplied by Metalastik Co. in England, were installed in each span. The lines have been in service since 1969 and are a major component of the transmission system, responsible for transmitting 85% of the total power to the converter station.

MH noted the first signs of trouble in 1987 when tangent tower crossarms failed. Later, in the early 1990s, MH noticed that spacer-dampers were displaced with arms locked out of their original positions. Full-size tower-head analysis by MH and stress analysis by the Industrial Technology Centre (ITC; Winnipeg, Manitoba) concluded that the failures resulted from bending fatigue. To determine the causes of this fatigue, MH engaged the Houston Company of Meaford, Ontario, in conjunction with the ITC, to conduct an analysis of the problem.

Finding a Solution

A sample of 25 in-situ and five as-new spacer-dampers, subjected to failure analysis, showed that the elastomeric bushing separating the clamp arm from the damper body was at the end of its useful life. In addition, the fasteners from the 25 field samples showed excessive corrosion damage. A test section of Bipole 1 and 2, which was used to study Aeolian conductor vibration, showed that increased vibration resulted from spacer-damper failures, which then provided sufficient energy to the tower crossarm members to increase the bending fatigue and reduce the life of the crossarm members. Predictions based on this testing indicated that 10% of the spacer-dampers could fail by 2001 and 50% by 2004.

MH determined it needed to rehabilitate the existing spacer-dampers by replacing the failed elastomeric bushings with upgraded ASTM A320-L7 fasteners, and by replacing bolts and washers for the connections between the clamp arm and conductor, and between the clamp arm and body. MH would reuse the castings, as they had proved their function and strength, had met corona inception criteria and were still in excellent condition. MH also determined it was necessary to modify the positioning and number of spacer-dampers to optimize the range of damping capability.

The immediate benefit of the rehabilitation, especially the respacing of the larger number of dampers, was an increase in the time to failure of the dampers from 12 years to more than 206 years, a 17-fold increase in damper life. The new spacing of the spacer-damper units provided increased damping of the sub-conductor oscillation and, thereby, increased protection from fatigue of the units. An additional benefit from the rehabilitation process resulted from the ability to perform certain line maintenance jobs involving the installation of conductor patch rods, replacement of conductor shoe cotter pins on every structure, and conducting a detailed examination of the line.

The Construction Process

Since it is mandatory that the lines remain energized at all times, MH was constrained to use hot-line techniques during the rehabilitation work. In addition, System Control Centre requirements dictate that no work be performed during July and August. With the summer months ruled out, the weather during the remaining seasons became a significant consideration in planning the work. The summer season in northern Manitoba is defined, for all practical purposes, as being between mid May and mid October. Work that is performed earlier or later may be delayed or halted due to winter conditions.

Access from Radisson tower 1 to tower 840, a distance of 248 miles (399 km) over swamp and woodland, is accessible only by air. The nearest land access is a paralleling railroad line about 25 miles (40 km) to the north by air. The section from tower 840 to tower 1380, a distance of 161 miles (259 km), also is over swamp and woodland, and accessible by air and a paralleling highway about 1 mile (1.6 km) to the west. From tower 1380 to Dorsey, a distance of 149 miles (240 km), the right-of-way consists woodland and cultivated farms and is accessed from a nearby road.

The project, publicly tendered with a request for a combination of helicopter and cart usage, was awarded to Comstock Canada and Canadian Helicopters Inc. (CCHL) in a joint venture. MH supplied 9000 new float dampers from Slater Steel Ltd. (Brantford, Ontario, Canada), as well as all new bushings and replacement suspension clamp cotter pins. The work procedure used a Eurocopter Twinstar helicopter modified with a fully insulated, electrically nonconducting fiberglass boom, which extended through the passenger compartment of the craft. Counterweights were attached to one end, if required; attached to the other end, at various times, were picker arms, a cart lifting arm or a passenger platform. Also used was a newly designed and fabricated hydraulically powered conductor cart equipped with orbit drive motors, lifting arms to move through suspension points and a mechanical counter.

The work was scheduled for 2001 from tower 840 from mid May to the end of June and commencing again in early September until the middle of October. For 2002, the work was scheduled to start at tower 1, working toward tower 840, and as in 2001, from mid May until the end of June and from early September to mid October.

The daily work procedure began with the issuance of safety hold-offs on all lines, placement of live-line trained personnel in carts and placement of new or refurbished dampers in baskets in each span. Carts moved through each span replacing dampers and leaving old dampers in baskets for removal by helicopter, which delivered baskets of new or refurbished dampers. Units that required rebuilding were taken to a central shop or mobile shop, depending on the location of the work. Cotter keys were replaced at every suspension point clamp, and the damaged conductor was repaired with armor rods.

MH personnel included one senior live-line journeyman to issue safety hold-offs and to act as a safety watch. One senior construction inspector acted as a safety watch and maintained a count of units removed and replaced. A senior construction supervisor maintained liaison with the contractor on project management issues, maintained progress statements and ensured that damper placement met specified tolerances.

Safety Measures

Since the first priority on any project is to ensure the safety of all personnel, MH asked the CCHL joint venture to submit a copy of its training and safety management program for review by MH's departments of Occupational Safety and Health, Live line Procedures and Transmission and Civil Construction. A project safety plan, which included some revisions of the CCHL safety program, was developed with the contractor covering safety hold-off requirements, helicopter safety, bonding and grounding procedures, conductor cart aerial rescue, safety around refurbishment shops and safety around landing areas for helicopters.

All linemen were required to have had at least three years of experience within the last five years as a transmission lineman for an electric utility or for a specialized contractor engaged in transmission maintenance for electric utilities. All linemen also passed a five-day, task-specific live-line training course and a three-day session for hands-on training in helicopter platform transfers, bonding procedures, conductor cart operation and aerial rescue. These training sessions were conducted at the start of every construction period.

Once the project work commenced, MH held daily tailboard discussions to get interaction between the contractor's personnel and representatives from MH. In all cases, good information was exchanged.

MH also scheduled biweekly safety meetings to address more general issues and to ensure full interaction among all personnel. This attention to safety resulted in no lost man-hours for the 31,000 man-hours worked.

Work Activities

With line, ground and MH crews on site by 7:15 a.m., safety hold-offs for all lines was in place by 7:30 and the daily tailboard meetings completed by 7:30. Conductor carts, which were tethered to each line overnight, were subjected to inspection and maintenance by 8:15 when ground crews moved out using six-wheel drive tracked all-terrain vehicles. The helicopters supplied the baskets of refurbished dampers ahead of the linemen, who removed old dampers and installed the refurbished units in new locations. The old dampers were retrieved by the helicopter crew after the linemen had replaced suspension clamp cotter keys before moving on to the next span.

On the ground, the MH senior inspector moved ahead of line personnel to count dampers being removed and, at the same time, inspected each tower for loose hardware using image stabilizer binoculars. Work was suspended until loose hardware was tightened. The senior live-line journeyman followed the line crew and, acting as safety watch at all times, observed all work with special attention to suspension crossings using his image stabilizer binoculars. The journeyman had authority to stop work at any time in the event of a safety infraction. In addition, two contractor ground crews worked about half-span ahead of the line crews to clear trees that may have posed a danger to the workers, while maintaining a safety watch and offering support to the line workers. In the meantime, MH's senior site supervisor followed with the line crew, recording the number of dampers installed. The line crews zeroed their counters at the start of every span and called in their counter readings as they completed the span. This distance was recorded and compared to a theoretical arc length distance for each span.

On a typical day, 720 dampers were removed and 800 installed. The refurbishing shop rebuilt about 700 a day, so the initial float of 9000 new dampers was used to maintain a high level of productivity in the air. Work continued until about 6:30 p.m., 7 days a week for about 35 to 40 days.

The conductor carts, capable of moving through all suspension points and light angle towers up to 12 degrees in deflection, could be lifted from the line if the angle was exceeded. In these cases, the line crew was ferried to the ground by helicopter, while the carts were lifted from the line and moved to the other side of the angle tower. This operation averaged about 1.5 hours to move all four carts and to get the linemen back in the air.

Problems and Solutions

MH had specified that dampers were to be placed in new locations within each span to an accuracy of ±6 inches (15 cm) from the edge of the suspension clamp to optimize the damper replacement program. The contractor proposed the use of a cart-mounted counter and a hard rubber measuring wheel, which would run along the conductor, to determine distances from the starting point. However, there was difficulty in matching the catenary curve distance measured by the counter with the MH checks using the horizontal distances, resulting in a work stoppage to settle the issue. The contractor then revised the counter mounts to the cart and replaced the hard rubber measuring wheel with an aluminum alloy knurled wheel to minimize wheel wear, since 0.005 inches (0.01 cm) in wear equates to a difference in measurement of 10 ft (3 m) for a 1600 ft (488 m) distance.

MH developed a system for comparing theoretical catenary curve distances from suspension point to suspension point with actual distances recorded by the counter, which was sufficiently accurate to ensure placement of dampers in accordance with specifications. When work resumed, three days worth of work was repeated to ensure that dampers were placed correctly.

Ground conditions were challenging in more than 60% of the line distance but were mitigated by the use of the all-terrain vehicles, which were mounted on rubber tracks and equipped with 2500-lb (1134-kg) winches.

In addition to rehabilitating the lines with new spacer locations, other benefits included the opportunity for: close visual examination and recording of conductor damage; hand clearing of trees in close proximity to the conductor; and visual examination of all insulator and connective hardware, which allowed for repairs to be made as the work progressed. In this connection, there were 18 locations where cotter keys on clevis assemblies between the bottom insulator and yoke plate were repositioned for greater security.

Acknowledgments

This article was originally presented in a technical session at the IEEE ESMO 2003 conference in Orlando, Florida, U.S.

Gary Smith joined Manitoba Hydro in 1980 and has held positions in surveys and mapping, generation and line construction. He is presently a senior supervisor in the transmission line construction area.

Shane Mailey received a BSCE degree from the University of Manitoba in 1991. He joined Manitoba Hydro that same year and has held positions in design and construction. He is presently section head/project engineer in the transmission line construction area.

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